An Apparatus and Associated Methods for Audio Presented as Spatial Audio

ABSTRACT

An apparatus, based on a first audio track of at least one audio track, the first audio track audibly presented to the user as spatial audio such that it is perceived to originate from a particular location and based on the user being within a predetermined distance of the particular location;configured to provide for a change in the audible presentation of the first audio track to the user from presentation as spatial audio to presentation as at least one of monophonic and stereophonic audio.

TECHNICAL FIELD

The present disclosure relates to the field of spatial audio and, inparticular, to the field of changing the way in which audio tracks areaudibly presented to a user, associated methods, computer programs andapparatus.

BACKGROUND

The augmentation of real-world environments with graphics and audio isbecoming common, with augmented/virtual reality content creatorsproviding more and more content for augmentation of the real-world aswell as for virtual environments. The presentation of audio as spatialaudio, which is such that the audio is perceived to originate from aparticular location, is useful for creating realistic augmented realityenvironments and virtual reality environments. The effective andefficient management of spatial audio for a user may be challenging.

The listing or discussion of a prior-published document or anybackground in this specification should not necessarily be taken as anacknowledgement that the document or background is part of the state ofthe art or is common general knowledge. One or more aspects/examples ofthe present disclosure may or may not address one or more of thebackground issues.

SUMMARY

In a first example aspect there is provided an apparatus comprising:

-   -   at least one processor; and    -   at least one memory including computer program code,    -   the at least one memory and the computer program code configured        to, with the at least one processor, cause the apparatus to        perform at least the following:    -   based on a first audio track of at least one audio track, the        first audio track audibly presented to the user as spatial audio        such that it is perceived to originate from a particular        location and based on the user being within a predetermined        distance of the particular location;    -   provide for a change in the audible presentation of the first        audio track to the user from presentation as spatial audio to        presentation as at least one of monophonic and stereophonic        audio.

In one or more examples, when the first audio track is presented asspatial audio, signalling indicative of movement of the user providesfor modification of one or more of the direction from which the audiotrack is perceived to originate relative to the user's head and itsvolume; and

-   -   when the first audio track is presented as at least one of        monophonic and stereophonic audio, signalling indicative of        movement of the user does not provide for modification of one or        more of the direction from which the audio track is perceived to        originate relative to the user's head and its volume.        Accordingly, in one or more examples, the first audio track may        be perceived by the user to change from being heard from the        particular location to being taken with the user as they move        away from the particular location. Thus, in one or more        examples, the user may perceive that they have “picked up” the        first audio track after they move away from the particular        location such that they can continue to listen to the audio of        the first audio track without the spatial audio effect as they        continue moving about in a virtual or real-world space.

In one or more embodiments, the first audio track is audibly presentedas spatial audio by processing the audio track using one or more of:

-   -   i) a head-related-transfer-function filtering technique;    -   ii) a vector-base-amplitude panning technique; and    -   iii) binaural audio presentation.

In one or more embodiments, the provision of the change to monophonic orstereophonic audio is based on the user being within a predetermineddistance of the particular location and providing a predefined userinput.

In one or more examples, the user input comprises at least one or moreof:

-   -   a) a head movement gesture to position an ear of the user        adjacent to or at the particular location;    -   b) a user hand gesture towards the particular location;    -   c) a voice command;    -   d) a user input via a user input terminal.

In one or more embodiments, the particular location and the location ofthe user is defined relative to one of:

-   -   a virtual space, the virtual space comprising a virtual        environment in which a representation of the user is free to        move; and    -   a real-world space in which the user is located.

In one or more embodiments, the user is associated with at least twospeakers, the two speakers located on opposed sides of the user's headto provide audio to different ears and wherein based on a side of thehead of the user that is closest to the particular location, at leastwithin a predetermined time window prior to provision of said change,defines, at least in part, which one of the at least two speakers thefirst audio track is provided to for audible presentation with a greatervolume than the other when the apparatus provides for said presentationof the first audio track as one of monophonic and stereophonic audio.

In one or more embodiments, based on the user being presented with asecond audio track as one of monophonic and stereophonic audio, theprovision of the change in the audible presentation of the first audiotrack provides for:

-   -   audible presentation of the first audio track with a first        speaker distribution profile; and    -   audible presentation of the second audio track with a second        speaker distribution profile different to the first speaker        distribution profile; wherein    -   the first and second speaker distribution profiles define at        least which of a first speaker and a second speaker the audio        track is presented to, the first and second speakers located on        opposed sides of the user's head to provide audio to the user's        left and right ears.

In one or more examples, the speaker distribution profiles areconfigured such that the apparatus is caused to provide a left stereochannel and a right stereo channel and, optionally, a centre stereochannel for use in presentation of the first audio track, the secondaudio tracks and any further audio tracks.

In one or more embodiments,

-   -   a) the second speaker distribution profile is the same speaker        distribution profile used to present the second audio track        prior to audible presentation of the first audio track as one of        monophonic and stereophonic audio;    -   b) the second speaker distribution profile is a different        speaker distribution profile used to present the second audio        track prior to audible presentation of the first audio track as        one of monophonic and stereophonic audio.

In one or more embodiments, a predetermined audio track limit of two orgreater is provided that limits the number of audio tracks that can bepresented to the user as one of monophonic and stereophonic audio at anyone time; and wherein

-   -   based on the user being presented with a maximum number of audio        tracks as one of monophonic and stereophonic audio that meets        said audio track limit; and    -   based on the provision of the change in the audible presentation        of the first audio track, the apparatus is caused to provide for        the ceasing of the audible presentation as one of monophonic and        stereophonic audio of at least one of the maximum number of        audio tracks.

In one or more examples, the apparatus is caused to maintain a list ofaudio tracks presented to the user as one of monophonic and stereophonicaudio at any one time. In one or more examples, a predetermined audiotrack limit of two or greater is provided that limits the number ofaudio tracks in said list. In one or more examples, the apparatus iscaused to provide for audible presentation of audio tracks in the listand not provide for audible presentation of audio tracks not in thelist, wherein the addition of an audio track to the list when the audiotrack limit has been reached provides for removal of an audio track fromthe list. The removal may be on a first in first out basis or may beuser-selected.

In one or more embodiments, the apparatus is caused to maintain a listof audio tracks for presentation to the user as one of monophonic andstereophonic audio at any one time, and the audio tracks in said listare user selectable such that on selection of one of the audio tracks inthe list, the apparatus is caused to provide for audible presentation ofthe selected audio track without unselected audio tracks in the list asone of monophonic and stereophonic audio.

In one or more examples, user selection may be provided by a usergesture, such as a head movement gesture or, more particularly, an up ordown head movement gesture.

In one or more examples, based on the apparatus being caused to providefor ceasing of the audible presentation of at least one of the maximumnumber of audio tracks and based on the user providing a user inputindicative of a desire to prevent the ceasing of the audiblepresentation, provide for incremental increasing of the audio tracklimit such that the first audio track is presented along with the saidaudio tracks in said list. In one or more examples, the user inputcomprises the user placing their hand adjacent or over one or both oftheir ears.

In one or more embodiments, based on signalling indicative of adetection of a predetermined head movement of the user, the apparatus iscaused to provide for ceasing of audible presentation of at least oneaudio track presented to the user as one of monophonic and stereophonicaudio.

In one or more examples, the predetermined head movement may comprisetilting of the user's head. The user may associate such a predeterminedhead movement with an imaginary action of tipping the audio track out oftheir head. In one or more examples, based on a plurality of audiotracks being presented to the user as one of monophonic and stereophonicaudio, the apparatus is configured to select which of the plurality ofaudio tracks ceases presentation based on a direction to which the usertilts their head as indicated in the signalling. In one or moreexamples, one of the plurality of audio tracks may be presentedprimarily to the left speaker and a second of the plurality of audiotracks may be presented primarily to the right speaker and therefore thedetection of tilting of the user's head to the left may provide forceasing presentation of the audio track presented to the left speakerand, likewise, detection of tilting of the user's head to the right mayprovide for ceasing presentation of the audio track presented to theright speaker.

In one or more embodiments, the first audio track comprises one of aplurality of audio tracks, the plurality of audio tracks audiblypresented to the user as spatial audio such that each is perceived tooriginate from a respective particular location, and based on a userinput indicative of a selection of the plurality of audio tracks;

-   -   provide for a change in the audible presentation of the        plurality of audio tracks to the user from presentation as        spatial audio to presentation as at least one of monophonic and        stereophonic audio.

In one or more examples, the user input may comprise a user gesturedirected to the respective particular locations at which the pluralityof audio tracks are perceived to originate.

In one or more examples, upon the change in the audible presentation ofthe plurality of audio tracks, a speaker distribution profile is definedfor each audio track, the speaker distribution profile defining at leastto what degree the audio track is presented to a first speaker and asecond speaker the audio, the first speaker configured to present audioto a left ear of the user and the second speaker configured to presentaudio to a right ear of the user, the speaker distribution profiledetermined based on the relative locations of the particular locationsof the plurality of audio tracks.

In one or more embodiments, based the user being within a predetermineduser-transfer distance of a second user, provide for generation ofsignalling to provide for one or more of

-   -   i) presentation of a shared audio track to the second user as at        least one of monophonic and stereophonic audio, the shared audio        track comprising an audio track presented to the first user as        at least one of monophonic and stereophonic audio; and    -   ii) presentation of a shared audio track to the first user as at        least one of monophonic and stereophonic audio, the shared audio        track comprising an audio track presented to the second user as        at least one of monophonic and stereophonic audio.

In one or more embodiments, the provision of the signalling is furtherbased on a user gesture from one or both of the user and the seconduser.

In a further aspect there is provided a method, the method comprising

-   -   based on a first audio track of at least one audio track, the        first audio track audibly presented to the user as spatial audio        such that it is perceived to originate from a particular        location and based on the user being within a predetermined        distance of the particular location;    -   providing for a change in the audible presentation of the first        audio track to the user from presentation as spatial audio to        presentation as at least one of monophonic and stereophonic        audio.

In a further aspect there is provided a computer readable mediumcomprising computer program code stored thereon, the computer readablemedium and computer program code being configured to, when run on atleast one processor, perform the method of:

-   -   based on a first audio track of at least one audio track, the        first audio track audibly presented to the user as spatial audio        such that it is perceived to originate from a particular        location and based on the user being within a predetermined        distance of the particular location;    -   providing for a change in the audible presentation of the first        audio track to the user from presentation as spatial audio to        presentation as at least one of monophonic and stereophonic        audio.

In a further aspect there is provided an apparatus, the apparatuscomprising means configured to;

-   -   based on a first audio track of at least one audio track, the        first audio track audibly presented to the user as spatial audio        such that it is perceived to originate from a particular        location and based on the user being within a predetermined        distance of the particular location;    -   provide for a change in the audible presentation of the first        audio track to the user from presentation as spatial audio to        presentation as at least one of monophonic and stereophonic        audio.

The present disclosure includes one or more corresponding aspects,examples or features in isolation or in various combinations whether ornot specifically stated (including claimed) in that combination or inisolation. Corresponding means and corresponding functional units (e.g.,function enabler, AR/VR graphic renderer, display device) for performingone or more of the discussed functions are also within the presentdisclosure.

Corresponding computer programs for implementing one or more of themethods disclosed are also within the present disclosure and encompassedby one or more of the described examples.

The above summary is intended to be merely exemplary and non-limiting.

BRIEF DESCRIPTION OF THE FIGURES

A description is now given, by way of example only, with reference tothe accompanying drawings, in which:

FIG. 1 illustrates an example apparatus for providing for a change inthe audible presentation of an audio track;

FIG. 2 shows an example of a user being presented with spatial audiofrom four virtual locations each having an audio track associatedtherewith;

FIG. 3 shows an example of the user moving to a position within apredetermined distance of the particular location of a third audio trackof the plurality of audio tracks and performing a gesture to lean theirleft ear towards it;

FIG. 4 shows an example of the audio track having changed frompresentation as spatial audio to presentation as stereophonic audio;

FIG. 5 shows the user having moved to a position within a predetermineddistance of a particular location associated with a fourth of the audiotracks and having leaned their right ear towards the particular locationof the fourth audio track;

FIG. 6 shows the user having moved to a position within a predetermineddistance of a second of the audio tracks and having leaned their leftear towards the particular location associated with the second audiotrack;

FIG. 7 shows the user having moved to a position within a predetermineddistance of the particular location of a first of the audio tracks andhaving leaned their left ear towards the location which has “pushed out”the audio track associated with the fourth audio source;

FIG. 8 shows a user before and after performing a user gesture to ceasepresentation of one of the stereophonically presented audio tracks;

FIG. 9 shows a user having performed a user input to collectively changea plurality audio tracks associated with a plurality of differentparticular locations to presentation as stereophonic audio;

FIG. 10 shows a first example of the sharing of audio tracks between afirst user and a second user;

FIG. 11 shows a second example of the sharing of audio tracks between afirst user and a second user;

FIG. 12 shows a third example of the sharing of audio tracks between afirst user and a second user that leads to the exchange stereophonicallypresented audio tracks;

FIG. 13 shows an example augmented reality view of a user with augmentedreality graphics to illustrate the number/presentation of the audiotracks as stereophonic/monophonic audio;

FIG. 14 shows a flowchart illustrating an example method; and

FIG. 15 shows a computer readable medium.

DESCRIPTION OF EXAMPLE ASPECTS

Virtual reality (VR) may use a VR display comprising a headset, such asglasses or goggles or virtual retinal display, or one or more displayscreens that surround a user to provide the user with an immersivevirtual experience. A virtual reality apparatus, which may or may notinclude the VR display, may provide for presentation of multimedia VRcontent representative of a virtual reality scene to a user to simulatethe user being present within the virtual reality scene. Accordingly, inone or more examples, the VR apparatus may provide signalling to a VRdisplay for display of the VR content to a user while in one or moreother examples, the VR apparatus may be part of the VR display, e.g.part of the headset. The virtual reality scene may therefore comprisethe VR content displayed within a three-dimensional virtual realityspace so that the user feels immersed in the scene, as if they werethere, and may look around the VR space at the VR content displayedaround them. The virtual reality scene may replicate a real world sceneto simulate the user being physically present at a real world locationor the virtual reality scene may be computer generated or a combinationof computer generated and real world multimedia content. Thus, the VRcontent may be considered to comprise the imagery (e.g. static or videoimagery), audio and/or accompanying data from which a virtual realityscene may be generated for display. The VR apparatus may thereforeprovide the VR scene by generating the virtual, three-dimensional, VRspace in which to display the VR content. The virtual reality scene maybe provided by a panoramic video (such as a panoramic live broadcast),comprising a video having a wide or 360° field of view (or more, such asabove and/or below a horizontally oriented field of view). A panoramicvideo may have a wide field of view in that it has a spatial extentgreater than a field of view of a user or greater than a field of viewwith which the panoramic video is intended to be displayed.

The VR content provided to the user may comprise live or recorded imagesof the real world, captured by a VR content capture device, for example.An example VR content capture device comprises a Nokia Technologies OZOdevice. As the VR scene is typically larger than a portion a user canview with the VR display, the VR apparatus may provide, for display onthe VR display, a virtual reality view of the VR scene to a user, the VRview showing only a spatial portion of the VR content that is viewableat any one time. The VR apparatus may provide for panning around of theVR view in the VR scene based on movement of a user's head and/or eyes.A VR content capture device may be configured to capture VR content fordisplay to one or more users. A VR content capture device may compriseone or more cameras and, optionally, one or more (e.g. directional)microphones configured to capture the surrounding visual and aural scenefrom a capture point of view. In some examples, the VR content capturedevice comprises multiple, physically separate cameras and/ormicrophones. Thus, a musical performance may be captured (and recorded)using a VR content capture device, which may be placed on stage, withthe performers moving around it or from the point of view of an audiencemember. In each case a consumer of the VR content may be able to lookaround using the VR display of the VR apparatus to experience theperformance at the capture location as if they were present.

Augmented reality (AR) may use an AR display, such as glasses or gogglesor a virtual retinal display, to augment a view of the real world (suchas seen through the glasses or goggles) with computer generated content.An augmented reality apparatus, which may or may not include an ARdisplay, may provide for presentation of multimedia AR contentconfigured to be overlaid over the user's view of the real-world. Thus,a user of augmented reality may be able to view the real worldenvironment around them, which is augmented or supplemented with contentprovided by the augmented reality apparatus, which may be overlaid ontheir view of the real world and/or aurally overlaid over an aural realworld scene they can hear. The content may comprise multimedia contentsuch as pictures, photographs, video, diagrams, textual information,aural content among others. Thus, while augmented reality may providefor direct viewing of the real world with the addition of computergenerated graphics and/or audio content, a user of virtual reality mayonly be able to see content presented on the VR display of the virtualreality apparatus substantially without direct viewing of the realworld.

In addition to the audio received from the microphone(s) of the VRcontent capture device further microphones each associated with adistinct audio source may be provided. In one or more examples, the VRcontent capture device may not have microphones and the aural scene maybe captured by microphones remote from the VR content capture device.Thus, microphones may be provided at one or more locations within thereal world scene captured by the VR content capture device, eachconfigured to capture audio from a distinct audio source. For example,using the musical performance example, a musical performer or apresenter may have a personal microphone. Knowledge of the location ofeach distinct audio source may be obtained by usingtransmitters/receivers or identification tags to track the position ofthe audio sources, such as relative to the VR content capture device, inthe scene captured by the VR content capture device. Thus, the VRcontent may comprise the visual imagery captured by one or more VRcontent capture devices and the audio captured by the one or more VRcontent capture devices and, optionally/alternatively, one or morefurther microphones. The location of the further microphones may beprovided for providing spatial audio.

Spatial audio comprises audio presented in such a way to a user that itis perceived to originate from a particular location, as if the sourceof the audio was located at that particular location. Thus, virtualreality content may be provided with spatial audio having directionalproperties, such that the audio is perceived to originate from a pointin the VR space, which may be linked to the imagery of the VR content.Augmented reality may be provided with spatial audio, such that thespatial audio is perceived as originating from real world objectsvisible to the user and/or from augmented reality graphics overlaid overthe user's view.

Spatial audio may be presented independently of visual virtual realityor visual augmented reality content. Nevertheless, spatial audio, insome examples, may be considered to be augmented reality content becauseit augments the aural scene perceived by a user. As an example ofindependent presentation of spatial audio, a user may wear headphonesand, as they explore the real world, they may be presented with spatialaudio such that the audio appears to originate at particular locationsassociated with real world objects or locations. For example, a citytour could be provided by a device that tracks the location of the userin the city and presents audio describing points of interest as spatialaudio such that the audio appears to originate from the point ofinterest around the user's location.

The spatial positioning of the spatial audio may be provided by 3D audioeffects, such as those that utilise a head related transfer function tocreate a spatial audio space in which audio can be positioned forpresentation to a user. Spatial audio may be presented by headphones byusing head-related-transfer-function (HRTF) filtering techniques or, forloudspeakers, by using vector-base-amplitude panning techniques toposition the perceived aural origin of the audio content. Spatial audiomay use one or more of volume differences, timing differences and pitchdifferences between audible presentation to each of a user's ears tocreate the perception that the origin of the audio is at a particularlocation in space.

In some examples, an audio track, which comprises audio content forpresentation to a user, may be provided for presentation as spatialaudio. Accordingly, the audio track may be associated with a particularlocation which defines where the user should perceive the audio of theaudio track as originating. The particular location may be definedrelative to a virtual space or a real-world space. The virtual space maycomprise a three-dimensional environment that at least partiallysurrounds the user and may be explorable by the user. The virtual spacemay be explorable in terms of the user being able to move about thevirtual space by at least translational movement based on user input. Ifthe spatial audio is provided with virtual reality content, virtualreality imagery may be displayed in the virtual space along with spatialaudio to create a virtual reality experience. If the spatial audio isprovided with visual augmented reality content or independently ofaugmented or virtual reality content, the particular location may bedefined relative to a location in the real world, such as in areal-world room or city.

As mentioned above, the audio track may be associated with locationinformation indicative of the particular location at which the audio ofthe audio track should be perceived as originating. In some examples,the location information may identify an object, which may or may not bemoveable, and which may be designated as the location from which thespatial audio should be perceived as originating. Thus, a real worldobject may be designated and when that real world object is identifiedin a space in which the user is located, the user may be presented withspatial audio such that they perceive the audio of the audio track tooriginate from said real world object. In some examples, the audio trackis not associated with location information and instead it is decided atthe time of presentation where to place the particular location definingthe origin of the spatial audio.

FIG. 1 shows an example system 100 for presentation of spatial audio toa user. The system 100 includes an example apparatus 101 for controllingthe presentation of audio tracks based on the user's location. Theapparatus 101 may comprise or be connected to a processor 101A and amemory 101B and may be configured to execute computer program code. Theapparatus 101 may have only one processor 101A and one memory 101B butit will be appreciated that other embodiments may utilise more than oneprocessor and/or more than one memory (e.g. same or differentprocessor/memory types). Further, the apparatus 101 may be anApplication Specific Integrated Circuit (ASIC).

The processor may be a general purpose processor dedicated toexecuting/processing information received from other components, such asfrom a location tracker 102 and a content store 103, in accordance withinstructions stored in the form of computer program code in the memory.The output signalling generated by such operations of the processor isprovided onwards to further components, such as to audio presentationequipment, such as headphones 108.

The memory 101B (not necessarily a single memory unit) is a computerreadable medium (solid state memory in this example, but may be othertypes of memory such as a hard drive, ROM, RAM, Flash or the like) thatstores computer program code. This computer program code storesinstructions that are executable by the processor, when the program codeis run on the processor. The internal connections between the memory andthe processor can be understood to, in one or more example embodiments,provide an active coupling between the processor and the memory to allowthe processor to access the computer program code stored on the memory.

In this example the respective processors and memories are electricallyconnected to one another internally to allow for electricalcommunication between the respective components. In this example thecomponents are all located proximate to one another so as to be formedtogether as an ASIC, in other words, so as to be integrated together asa single chip/circuit that can be installed into an electronic device.In some examples one or more or all of the components may be locatedseparately from one another.

The apparatus 101, in this example, forms part of an augmented realityapparatus 104 for presenting visual imagery in augmented reality. In oneor more other examples, the apparatus 101 may form part of a VRapparatus. In one or more examples, the apparatus 100 may be independentof an AR or VR apparatus and may provide signalling to audiopresentation equipment 108 (such as speakers, which may be incorporatedin headphones) for presenting the audio to the user. In this example,the processor 101A and memory 101B is shared by the AR apparatus 104 andthe apparatus 101, but in other examples, they may have their ownprocessors and/or memory.

The AR apparatus 104 may provide for display of one or more augmentedreality graphics over the user's view using an AR headset 107. In one ormore examples in which the apparatus 100 is independent of an AR or VRapparatus the AR headset may not be required and instead only the audiopresentation equipment 108 may be provided.

The apparatus 101 or the AR apparatus 104 under the control of theapparatus 101 may provide for aural presentation of audio to the userusing the headphones 108. The apparatus 101 may be configured to processthe audio such that, at any one time, it is presented as one of spatial,monophonic and stereophonic audio or, alternatively or in addition, theapparatus 101 may provide signalling to control the processing and/orpresentation of the audio.

Accordingly, an audio processor (not shown) may perform the audioprocessing in order to present the audio in the ways mentioned above.

The apparatus 101 may receive signalling indicative of the location ofthe user from a location tracker 102. The location tracker 102 maydetermine the user's location in the real world so that it may becompared to the particular location(s) from which spatial audio isperceived to originate. If the spatial audio is provided in a virtualreality environment, the location tracker may provide signallingindicative of the user's virtual location in the virtual space in whichthe virtual reality experience is provided so that it may be compared tothe virtual particular location(s) from which spatial audio is perceivedto originate.

In the examples that follow, the one or more audio tracks, which may beprovided from content store 103, may be processed such that they arepresented to the user as spatial audio or stereophonic or monophonicaudio. Accordingly, in a first instance, the audio track may bepresented as spatial audio and as such may undergo audio processing suchthat it is perceived to originate from a particular location. In asecond instance, the same audio track may be presented as monophonicaudio and as such may undergo audio processing (if required) such thatthe audio is presented monophonically to one or both of a left and rightspeaker associated with the left and right ears of the user. In a thirdinstance, the same audio track may be presented as stereophonic audio(if required) and as such may undergo audio processing such that theaudio of the audio track is presented to one or both of a left and rightspeaker associated with the left and right ear of the user respectively.Monophonic audio, when presented to two speakers provides the same audioto both ears. Stereophonic audio may define two (left and right) orthree (left, right, centre) stereo audio channels and the audio of theaudio track may be presented to one or more of those channels. In someexamples, the difference between stereophonic presentation and spatialaudio presentation may be, for spatial audio, the use of a time delaybetween corresponding audio being presented to speakers associated witha respective left and right ear of the user and, for stereophonicpresentation, the non-use of said time delay. It will be appreciatedthat the presentation of spatial audio may additionally use otherpresentation effects in addition to differences in the time thatcorresponding portions of the audio is presented to the user's ears tocreate the perception of a direction or location from which the audio isheard, such as volume differences amongst others.

While the same audio track may undergo audio processing in order toprovide for its presentation as spatial audio or stereophonic ormonophonic audio, as described above, in one or more other examples, theaudio tracks may be pre-processed and may thus include differentversions for presentation as spatial audio or stereophonic or monophonicaudio.

FIG. 2 shows a plan view of a real-world environment with a user 200wearing the headphones 108. In this example, the user is presented withfour audio tracks as spatial audio.

In the figures that follow an icon is used to represent each audiotrack, which are positioned at the locations from which the user 200perceives them to originate. A first audio track 201 is presented asspatial audio such that it is perceived to originate at a firstparticular location marked as 201L. A second audio track 202 ispresented as spatial audio such that it is perceived to originate at asecond particular location marked as 202L. A third audio track 203 ispresented as spatial audio such that it is perceived to originate at athird particular location marked as 203L. A fourth audio track 204 ispresented as spatial audio such that it is perceived to originate at afourth particular location marked as 204L. The pattern used in thefigure may be used to distinguish the audio tracks 201-204 that ispresented at the particular locations 201 L, 202L, 203L, 204L. As theaudio tracks are presented as spatial audio, the user 200 will perceivethe first audio track to be heard from a direction front and left oftheir head; the second audio track will be heard from a direction frontand slightly off centre to the left; the third audio track will be heardfrom a direction front and slightly off centre to the right; and thefourth audio track to be heard from a direction front and right of theirhead.

As an example, the audio tracks may comprise an audio description ofdifferent points of interest in the real-world environment and the usermay be hearing a description of at least one of them. As anotherexample, the audio tracks may comprise audio of a different instrumentof an orchestra. Thus, the user may be touring a concert venue that maybe physically absent of an orchestra but may hear a virtual orchestraand therefore the first to fourth particular locations may correspond tothe locations of the virtual orchestra members, simulating a real worldexperience of the orchestra.

The user is free to explore the real world environment (obviouslysubject to physical barriers or rights of way) and may thereforeapproach the first through fourth particular locations 201L-204L. Inother embodiments, where the user explores a virtual reality spacerather than a real world space, the user may have a free view point inthe virtual reality space, which may be known as six degrees of freedomVR or volumetric VR to those skilled in the art. Thus, the particularlocation of the presentation of the spatial audio may be definedrelative to the real world or a virtual space depending on whether theuser is experiencing the spatial audio independently of an AR or VRexperience or as part of a VR experience. Accordingly, the user beingwithin a predetermined distance of the particular location may beinterpreted as being within a predetermined real-world distance orwithin a predetermined virtual distance in the virtual space. Theprinciples of the example embodiments provided herein thus apply to (i)a user physically moving around a real world environment which isaugmented with spatial audio;

and (ii) a user provided with a view of a virtual reality space, invirtual reality, with the spatial audio provided at particular locationsin the virtual reality space, the user providing user input to moveabout in the virtual reality space and accordingly provided with acorresponding view of the virtual reality space based on the user input.

As will be appreciated when the audio is presented as spatial audio,when the user changes their location in the real world environment,there is a corresponding change in the presentation of audio track asspatial audio. For example, the volume of the audio track presented tothe user may be a function of the distance of the user from thecorresponding particular location. Thus, in one or more examples, as theuser moves towards the particular location the spatial audio presentedaudio track is presented louder and as the user moves away the audiotrack is presented more quietly. Also, as the user moves their head, thedirection (relative to the user's head) from which the spatial audio isperceived to originate changes in accordance with the direction to theparticular location relative to the user's direction of view.

The user may wish to listen to one or more of the audio tracks whilethey continue to explore the real-world environment rather thatremaining near the particular location. In one or more examples, theapparatus 101 may provide a way for the user to collect or pick up audiotrack(s) that are presented as spatial audio in the space in which theuser is located. In one or more examples, the audio processing requiredto present an audio track as spatial audio may affect its quality orfidelity compared to stereophonic or monophonic presentation.Accordingly, stereophonic or monophonic presentation of the audio trackmay provide for an improved experience when listening to said audio insome situations. The apparatus 101 may provide a way for the user tolisten to audio tracks presented as spatial audio with improved qualityor fidelity.

The example of FIG. 3 shows the user having approached the thirdparticular location 203L and arrived at a position within apredetermined distance 300 of the third particular location 203L. Theparticular locations 201L-204L, in one or more embodiments, may bemarked by an augmented reality graphic, which the user may see throughthe AR headset 107. In other embodiments, no augmented reality graphicmay be provided and the user may rely on their aural perception of theparticular locations in order to navigate to a location within thepredetermined distance of them. If the user were exploring a VR space,the particular locations likewise may or may not be marked with a visualgraphic.

In this example the apparatus 101, based on the third audio track 203being audibly presented to the user as spatial audio (as shown in FIG.2) such that it is perceived to originate from a particular location203L and based on the user being within the predetermined distance 300of the particular location 203L; provides for a change in the audiblepresentation of the third audio track 203 to the user 200 frompresentation as spatial audio to presentation as at least one ofmonophonic and stereophonic audio (as shown in FIG. 4, described below).

Thus, in the example of FIG. 3, the apparatus 101 may determine that theuser 200 is desirous of “picking up” the third audio track 203 by virtueof the signalling from the location tracker 102 indicating that the useris within the predetermined distance 300 of the third particularlocation. The third particular location 203L may be known by or reportedto the apparatus 101 by the AR apparatus 104 or whatever other apparatusis responsible for presenting spatial audio. The predetermined distancemay be less than 0.5, 1, 2, 3, 4 or 5 metres. The apparatus 101 maydefine the predetermined distance based on how close or how many otherparticular locations for other spatial audio tracks are nearby. Thus,for a crowded area, the predetermined distance may be smaller than foran uncrowded area, so that it is clearer which audio track the userintends to interact with. The predetermined distance may be visuallypresented to the user 200, such as via AR display 107 by the apparatus101, so that the user 200 can see how close they need to get to theparticular location.

In the example of FIG. 4, the third audio track 203 is no longerpresented as spatial audio and is instead presented as stereophonicaudio to a left stereophonic channel associated with a left ear speaker401 of left and right ear speakers 401 and 402. FIG. 4 shows the iconhaving the diagonal patterning which represents the third audio track203 located in the user's head towards the left headphone speakerthereby indicating that it is presented stereophonically to (primarily)the left speaker.

It will be appreciated that in other embodiments, the audio of the thirdaudio track 203 may be presented monophonically to both speakers 401 and402. Alternatively, the third audio track may be presented to any one ofthe two or three stereophonic channels.

The user 200 may thereby be considered to have “picked up” the thirdaudio track in that they may move away from the particular location andpresentation of the third audio track may not be subject to the spatialaudio modifications to presentation of its volume/perceived direction.Thus, the volume of the third audio track may not be a function ofdistance from the third particular location 203 when presented asstereophonic audio or monophonic audio while when presented as spatialaudio it was. Further, when the user 200 looks around the space an audiotrack presented as spatial audio will be perceived to originate from thesame direction in the space relative to the user 200 but when presentedstereophonically/monophonically presentation of the audio track may beindependent of the head orientation of the user 200. Thus, whilepresenting the third audio track 203 as spatial audio, signallingindicative of movement of the user would provide for modification of oneor more of the direction from which the audio track is perceived tooriginate relative to the user's head and its volume. However, now thatthe third audio track is presented as stereophonic audio (as shown inFIG. 4), signalling indicative of movement of the user does not providefor modification of one or more of the direction from which the audiotrack is perceived to originate relative to the user's head and itsvolume.

In this embodiment, the ear or side of the head of the user 200 closestto the particular location when the user is within the predetermineddistance 300 is used to determine, at least in part, to which ear theaudio track is presented stereophonically. Thus, as shown in FIG. 3, theuser 200 leant towards the third particular location with their left earand, accordingly, the apparatus 101 is configured to change presentationof the third audio track 203 from spatial audio to stereophonicpresentation to the left channel speaker or ear of the user 200. In oneor more examples, the apparatus may be caused to provide forstereophonic audio presentation as follows:

-   -   a) determination that a user approached the particular location        with the left side of their head may provide for presentation to        the left stereo audio channel;    -   b) determination that a user approached the particular location        with the right side of their head may provide for presentation        to the right stereo audio channel; and optionally,    -   c) determination that a user approached the particular location        straight-on or without a substantial orientation to the left or        right side of their head may provide for presentation to the        centre stereo audio channel.

In one or more examples, the side of the head of the user 200 that isclosest to the particular location within a predetermined time windowaround when the apparatus 101 provides for said change in audiblepresentation, defines, at least in part, which speaker 401, 402 thecorresponding audio track is provided to for audible presentation. Thetime window may extend a period before and/or after the user moveswithin the predetermined distance 300. In one or more examples, atechnical effect of the time window may be to overcome the user 200being unable to exactly position their head at the time the user iswithin the predetermined distance 300. Thus, the apparatus 101 may becaused to determine, based on user head orientation data, which may bereceived from location tracker 102 or other apparatus, which side of thehead the user 200 is closest to the third predetermined location 203Land provide for stereophonic presentation to the stereo channelcorresponding to the determined side.

The predetermined time window may extend from a time when the user movesfrom outside to within the predetermined distance 300. In one or moreexamples, the apparatus 101 may determine, in a time window of 1, 2 ormore seconds, which, if any, side of the user's head is directed towardsthe particular location 203L and then provide for said change in audiblepresentation. In one or more examples, the time window ends when theuser moves from outside to within the predetermined distance 300 andthus, the apparatus 101 is caused to determine which, if any, side ofthe user's head is directed towards the particular location 203L as theyapproach and reach the predetermined distance 300. The apparatus may usethe predetermined time window as a user input opportunity period inwhich the apparatus 101 identifies the occurrence of a user head gestureto present one side of the user's head to the particular location. Inother examples, the apparatus may average the user's head orientationrelative to the particular location over the predetermined time windowto determine if the user 200 presented one side of the user's head tothe particular location.

In some examples, the audio track may be presented exclusively to theear of the user closest to the particular location when the user waswithin the predetermined distance. However, more generally, the audiotrack may be presented to the closest speaker at a greater volume thanthe other of the left and right speakers.

It will be appreciated that in other examples, the apparatus 101 may notdetermine which ear is closest to the particular location and mayinstead, on provision of the change in audio presentation, provide forpresentation of the audio track to one or more of the stereophonic audiochannels (left, right or centre, for example) based on one or morefactors, such as a predetermined order, a user-preference order, and thepresence of other audio tracks presented stereophonically to the user.

In one or more examples, to provide the change in audible presentationthe apparatus may provide for ending of presentation of the third audiotrack 203 with one or more of:

-   -   i) a head-related-transfer-function filtering technique; and    -   ii) a vector-base-amplitude panning technique; and may provide        for presentation of the third audio track to said headphones or        speakers without these spatial audio processing techniques. This        may provide an improvement in quality of the audio due to the        lack of spatial audio processing.

While not shown in the example of FIG. 3, in one or more examples, theapparatus 101 may provide for the change to monophonic or stereophonicaudio based on the user being within the predetermined distance 300 ofthe particular location and providing a predefined user input. Thus,just being within the predetermined distance of the particular locationof the audio track may not cause the apparatus 101 to change the audiblepresentation of the associated audio track but on receipt of a userinput indicative of a desire to “pick up” the audio track, the apparatusmay provide for the change to stereophonic/monophonic presentation.

The user input may comprise a head movement gesture to position an earof the user adjacent towards or at the particular location. Thus, a userinput comprising a head gesture or leaning the head or ear towards theparticular location may be defined as the user input. In other examples,the user input may comprise a user hand gesture towards the particularlocation.

In another example, the apparatus 101 may be caused to make the changein audible presentation if it additionally determines that apredetermined voice command has been issued. In another example, theuser input may be provided via a smartphone, remote control or otheruser input terminal. In these examples, the user may approach aparticular location and indicate their desire for the apparatus 101 tochange presentation of the third audio track to one of stereophonic ormonophonic audio by providing the user input to said user inputterminal.

The example of FIG. 4 also shows the user approaching the particularlocation 204L of the fourth audio track 204. However, the user 200already has the third audio track 203 presented to them as stereophonicaudio to the left stereo channel. In one or more examples, the apparatus101, based on the user being already presented with the third audiotrack as stereophonic audio, and moving to a location within thepredetermined distance 300 of the fourth particular location 204L mayprovide for additional stereophonic or mixed monophonic presentation ofthe fourth audio track 204 along with the third audio track 203. Thus,for stereophonic presentation, the fourth audio track 204 may bepresented to one of the other channels e.g. right stereo channel orcentre stereo channel. Presentation on the left channel may result inexclusive presentation to a left ear of the user. Likewise, presentationon the right channel may result in exclusive presentation to a right earof the user. In one or more examples, presentation on the centre channelmay provide for presentation to the left and right ears equally. Thus,the additional audible presentation of the fourth audio track by theapparatus 101 may comprise the presentation on one other stereophonicchannel to that used for the third audio track 203.

Thus, for stereophonic presentation of two or more audio tracks, theapparatus may provide for assigning of the audio tracks to differentstereo channels. For monophonic presentation of two or more audio tracksthe apparatus 101 may provide for mixing of the audio tracks andsimultaneous presentation to one or both ears of the user.

In more general terms, for stereophonic presentation of multiple audiotracks (third 203 and fourth 204 in this example),

-   -   audible presentation of the third audio track 203 may be made        with a first speaker distribution profile; and    -   audible presentation of the fourth audio track 204 may be made        with a second speaker distribution profile different to the        first speaker distribution profile; wherein    -   the first and second speaker distribution profiles define at        least which of a first speaker 401 and a second speaker 402 the        audio of the respective audio track is presented to greater than        the other, the first speaker 401 configured to present audio to        a left ear of the user and the second speaker 402 configured to        present audio to a right ear of the user 200.

Thus, in one or more examples the speaker distribution profiles may beconfigured to define the left/right or left/right/centre stereo channelsmentioned above. Accordingly, a first speaker distribution profile maydefine a left stereo channel by defining 100% presentation to the leftstereo channel and 0% to the right stereo channel. A second speakerdistribution profile may define a right stereo channel by defining 100%presentation to the right stereo channel and 0% to the left stereochannel. A third speaker distribution profile may define a centre stereochannel by defining presentation to the left and right stereo channelssimultaneously, possibly with 50% volume to the left and 50% volume tothe right. In other examples, the speaker distribution profiles maydefine the volume the audio track is presented to each speaker orchannel ranging from zero to a maximum volume. Thus, four or more audiotracks may be presented by use of speaker distribution profilescomprising exclusively left speaker/exclusively right speaker andfurther speaker distribution profiles wherein the volume presented toone of the speakers is greater than the other by different amounts, forexample.

In one or more examples, the apparatus 101 provides for audiblepresentation of the audio tracks stereophonically by assignment of eachaudio track to one of a plurality of predefined audio track slots, eachslot defined by a different predetermined speaker distribution profile.

In the example of FIG. 4, the user 200 has moved from outside thepredetermined distance 300 to within the predetermined distance 300 withtheir right ear nearer to the particular location 204L of the fourthaudio track 204.

Thus, with reference to the example shown in FIG. 5, the apparatus 101has provided for stereophonic presentation of the fourth audio track tothe right stereo channel and thus to the user's right ear. As shown inFIG. 5, the third audio track 203 shown with diagonal patterning isillustrated in the left side of the user's head and the fourth audiotrack 204 is shown with uniform shading is illustrated in the right sideof the user's head to illustrate the sides to which the respectivespeaker distribution profiles are biased.

In the example of FIG. 6 the user has approached the second particularlocation 202L with their left ear towards the second particular location202L. However, in this example, the third audio track is alreadypresented to the left stereo channel/ear, as shown in FIG. 5. In one ormore examples, not shown in the figures, the apparatus 101 may providefor presentation of the second audio track to the centre stereo channel,which comprises a “spare” stereo channel on which no audio track iscurrently presented. However, this may not be what the user expectedgiven they approached the second particular location with their leftear.

In FIG. 6, the apparatus 101, based on a user request, either by way ofhead orientation on approach to the second particular location of thesecond audio track or otherwise, has been caused to provide forstereophonic presentation to a slot or stereo channel that already hasassociated with it a different audio track, and has assigned one or morealready-presented audio tracks to a different slot or stereo channel tomake way for presentation of the second audio track to the desired slotor stereo channel indicated by the user request. Thus, the third audiotrack is no longer presented on the left stereo channel and is presentedon the centre stereo channel, as illustrated by the diagonal patternedspot being positioned in the centre of the user's head. By reconfiguringwhich stereo channels or slots the “already stereophonically presented”audio tracks are assigned, the apparatus 101 provides an empty channelor slot for use in presentation of the “new” second audio track 202.

Thus, in this example, the second audio track is stereophonicallypresented to the left stereo channel, the third audio track has beenmoved to the centre channel and the fourth audio track is presented tothe right stereo channel.

The apparatus 101 may be caused to provide an audio track limitconfigured limit the number of audio tracks that may be presentedmonophonically or stereophonically simultaneously. For example, forthree channel stereophonic presentation the audio track limit may bethree wherein each audio track is given a different stereo channel andno further audio tracks can then be presented simultaneously.Alternatively, if the audio tracks are presented monophonically theaudio track limit may limit the number of audio tracks that are mixedtogether.

If the limit is reached, the apparatus 101 may not provide for changingof a further audio track from spatial audio presentation to one ofstereophonic and monophonic presentation when the user is within thepredetermined distance of its associated particular location. Theapparatus may provide for presentation of a warning to the user toinform them that the audio track limit has been reached. The user mayneed to provide a user input to select which of the audio trackspresented stereophonically/monophonically should cease being presented.

In this example, the audio track limit is three. Thus, in the example ofFIG. 6, the apparatus 101 has provided for the presentation of themaximum number of audio tracks as one of monophonic and stereophonicaudio up to said audio track limit.

FIG. 7 shows the user having moved to a location within thepredetermined distance 300 of the first particular location 201L withtheir left ear closer to the particular location. Accordingly, theapparatus 101 provided for the change in the audible presentation of thefirst audio track from spatial audio to stereophonic audio. The firstaudio track has been assigned the left stereo channel in accordance withthe user's left ear being identified as closest to the predeterminedlocation. However, because the audio track limit is reached, theapparatus 101 has also provided for the ceasing of the stereophonic ormonophonic audible presentation of at least one of the currentlypresented audio tracks, namely the fourth audio track in this example.The second and third audio tracks have been assigned to different stereochannels in order to vacate the left stereo channel for stereophonicpresentation of the first audio track. Thus, based on stereophonicpresentation of the first audio track to one of a left-most andright-most channel or slot, the apparatus may provide for ceasing ofstereophonic presentation of the audio track provided on an oppositechannel comprising the other of the left-most and right-most channel orslot. Additionally, the apparatus 101 may provide for shifting of theremaining audio channels to channels/slots that are more biased towardssaid other of the left-most and right-most channel or slot. This may beperceived by the user as adding the first audio track to their left earhas “pushed out” the audio track presented to their right ear, with theremaining audio tracks being shifted along towards the right ear.

In one or more examples, the apparatus may determine which channel orslot to present a currently-spatial-audio presented audio track when theaudio track limit has been reached and the user is within thepredetermined distance of the particular location of thecurrently-spatial-audio presented audio track based on user input. Inone or more examples, the apparatus may determine which channel or slotto present a currently-spatial-audio presented audio track when theaudio track limit has been reached based on user input. Thus, when theaudio track limit has been reached, rather than use the ear closest tothe particular location as an indicator of the channel or slot, theapparatus may be configured to select which slot or channel to presentthe currently-spatial-audio presented audio track based on user input.The apparatus, may further provide for ceasing stereophonic presentationof any audio track currently presented on the slot or channel selectedby the user, to make way for the stereophonic presentation of thecurrently-spatial-audio presented audio track.

Further, in one or more examples, the user may select whichstereophonically presented audio track to cease presentation of, so thata new audio track having a predetermined location within thepredetermined distance of the user can be presented to any one slot orchannel with the other stereophonically presented audio tracks beingrearranged over the channels or slots as appropriate. In one or moreexamples, based on the user being within a predetermined distance of theparticular location of where the audio of a further audio track isperceived to originate when the audio track limit is reached and basedon user selection of one or more of the audio tracks already presentedto the user as stereophonic audio, provide for ceasing of stereophonicpresentation of the user-selected audio tracks and for stereophonicpresentation of the further audio track. The further audio track may bepresented on the same channel or slot as the user-selected audio trackor a different one.

While the apparatus 101 may be caused to cease presentation of thefourth audio track as stereophonic audio, the fourth audio track may bepresented once again as spatial audio from its fourth particularlocation 204L. It will be appreciated that the user 200 may have moved asufficient distance away from the fourth particular location 204L andmay therefore no longer be able to hear the fourth audio track (if itwere presented at all). If the user is close enough to the fourthparticular location 204L, the apparatus 101 may provide for presentationof the fourth audio track as spatial audio based on cessation of itspresentation as stereophonic audio.

In one or more examples, the apparatus may provide for modification ofthe particular location 204L of the fourth audio track from the locationwhere the user “picked up” the audio track and thus caused it to bepresented stereophonically, to a new particular location (not shown inthe figures) based on the location of the user when the apparatus wascaused to cease presentation of the fourth audio track. Thus, ingeneral, when a stereophonically presented audio track ceasesstereophonic presentation, the apparatus may provide for audiblepresentation of the audio track as spatial audio with a particularlocation defined by the user's location at the time of ceasingstereophonic presentation. Accordingly, this may appear to the user ashaving “dropped” the audio at or near the user's current location and,subsequently, when they move away it is heard from the dropped locationas spatial audio.

It will be appreciated that when the audio track limit is reached therules followed by the apparatus 101 for selection of an audio track tocease presenting such that a new audio track can be presentedmonophonically or stereophonically may vary. For examples the apparatusmay:

-   -   i) cease stereophonic/monophonic presentation of a first-in        audio track comprising the audio track that was first presented        monophonically or stereophonically out of the audio tracks        currently presented in such a manner (this may be considered a        first-in-first-out selection);    -   ii) provide for the requesting of user input to select an audio        track for which to cease stereophonic/monophonic presentation;    -   iii) based on selection of the left or right side to present a        “new” audio track, cease stereophonic/monophonic presentation of        an opposite-side audio track comprising the audio track that is        presented monophonically or stereophonically on a stereo channel        or slot that is presented with most bias to an opposite speaker        to the selected side.

In the above examples, the audio tracks presented monophonically orstereophonically are presented simultaneously to different channels orslots. However, in other embodiments, the audio tracks provided by theapparatus 101 for presentation monophonically or stereophonically may bemaintained in a list of audio tracks for individual presentation to theuser or for presentation of a subset of said audio tracks in the list.The audio tracks in said list may be user selectable such that onselection of one of the audio tracks in the list, the apparatus iscaused to provide for audible presentation of the selected audio trackwithout the other unselected audio tracks in the list as one ofmonophonic and stereophonic audio.

The user selection of audio tracks from the list may be provided by auser gesture, such as a head movement gesture, a hand gesture, a inputvia a user input terminal or in any other appropriate way. In one ormore examples, a user input such as an up or down head movement gesturemay provide for sequential selection of one of the audio tracks in thelist.

In some examples, the audio track limit may be inconvenient to the user.For example, the user may not want one of the audio tracks presentedmonophonically or stereophonically to cease presentation uponpresentation monophonically or stereophonically of a further audio trackthat would exceed the audio track limit. Accordingly, the apparatus 101may, based on the audio track limit being exceeded and based on the userproviding a user input indicative of a desire to prevent the ceasing ofthe audible presentation of one of the audio tracks, provide forincremental increasing of the audio track limit such that the furtheraudio track is presented along with the said audio tracks alreadypresented monophonically or stereophonically. In one or more examples,the user input comprises the user placing their hand adjacent or overone or both of their ears. Such a gesture may be intuitive as it may beconsidered to represent the user holding the audio in their head againstthe default action of the apparatus 101 to ceasemonophonically/stereophonically presentation of one of the audio tracks.The apparatus may receive user gesture information from a gesturetracker to determine the occurrence of user input. The increasing of theaudio track limit may provide for definition, by the apparatus 101 of anadditional slot in which to present the further audio track or in whichto re-assign the other audio tracks.

FIG. 8 shows the apparatus 101 providing for user initiated ceasing ofthe presentation of one or more of the audio tracks that are presentedmonophonically/stereophonically. The left-hand side of FIG. 8 shows theuser 200 with the first audio track presented to the right stereochannel, the second audio track presented to the centre stereo channeland the third audio track presented to the left stereo channel. Theright-hand side of FIG. 8 shows the user having performed a user gesturecomprising a tilt of their head to the left. The apparatus 101, based onsignalling indicative of a detection of a predetermined head movement ofthe user to the left, may provide for ceasing of audible presentation ofat least one audio track presented to the user as one of monophonic andstereophonic audio on the corresponding left stereo audio channel. Thus,such a gesture may be considered as “tipping out” the (third) audiotrack from the user's head that is presented on the (left) side theyperform the tilting gesture. It will be appreciated that other headgestures, other gestures in general or other user input may be used toprovide for user initiated ceasing of the presentation of one or more ofthe audio tracks that are presented monophonically/stereophonically

The stereo audio channels or slots to which the audio tracks aremonophonically/stereophonically presented may depend on the number ofaudio tracks presented monophonically/stereophonically. Thus, forexample, FIG. 8 shows, after the user input provides for “tipping out”and therefore cessation of presentation of the third audio track, one ormore of the other, remaining monophonically/stereophonically presentedaudio tracks being redistributed amongst the stereo channels/slots.Thus, in this example, the first audio track remains presented to theright stereo channel and the second audio track is reassigned to theleft stereo channel from the centre stereo channel.

In the above examples, the “picking up” of single audio tracks isprovided by the user being within the predetermined distance 300 of theparticular location from which the spatially audio presented audiotracks is perceived to originate. FIG. 9 illustrates the apparatus 101providing for the “picking up” of a plurality of audio tracks in oneaction rather than individually approaching the particular location ofeach of them.

With reference to the example shown in FIG. 9, the plurality of audiotracks are audibly presented to the user 200 as spatial audio such thateach is perceived to originate from a respective particular location201L, 202L, 203L, 204L. The apparatus 101, based on signallingindicative of a user input to select the first through fourth audiotracks 201-204 may be caused to provide for a change in the audiblepresentation of the plurality of audio tracks to the user 200 frompresentation as spatial audio to presentation as at least one ofmonophonic and stereophonic audio. In particular, the user gesture maycomprise a hand gesture, a head gesture or a selection input on ahand-held or other user input terminal (not shown). In one or moreexamples, the user input may comprise a user gesture directed to therespective particular locations at which the plurality of audio tracksare perceived to originate, such as wave of the hand.

The plurality of audio tracks 201-204 may then each be assigned to adifferent stereo audio channel or different slot. The stereo audiochannel or slot assigned to each audio track may be based on therelative positions of the particular locations 201L, 202L, 203L, 204L inspace. Thus, as shown in FIG. 9, the left-most particular location (thefirst particular location 201L) is assigned to the left stereo channelor slot with the left-most bias in its speaker distribution profile forpresentation to the left and right speakers. Likewise, the right-mostparticular location (the fourth particular location 204L) is assigned tothe right stereo channel or slot with the right-most bias in its speakerdistribution profile for presentation to the left and right speakers.The second audio track and third audio track, being associated with theparticular locations 202L, 203L between the left-most and right-mostparticular locations are assigned slots with speaker distributionprofiles having a left/right speaker bias between the left-most bias andthe right-most bias. The arrows 901-904 show the assignment of the firstthrough fourth audio tracks each to one of four audio slots forstereophonic presentation.

In other examples, the apparatus 101 may provide for monophonicpresentation of a mix of the first through fourth audio tracks based onthe user input to select the first through fourth audio tracks.

The examples of FIGS. 10 to 12 illustrate the operation of the apparatus101 to share audio tracks that are presented as one of monophonic andstereophonic audio to one user to another user.

The example of FIG. 10 shows a first user 1001 and a second user 1002before and after sharing audio tracks. The first user 1001 and thesecond user 1002 may each be experiencing audio tracks presented asmonophonic/stereophonic audio and, optionally, other audio trackspresented as spatial audio. Accordingly, each user 1001, 1002 may have arespective apparatus 101 and AR apparatus 104 to provide the experienceexemplified above or a single apparatus 101 may provide for theexperience for both users.

The first user 1001 is presented with the first audio track 201stereophonically to the right stereo channel and the second audio track202 stereophonically to the left stereo channel. The second user 1002 ispresented with the third audio track 203 stereophonically to the leftstereo channel and the fourth audio track 204 stereophonically to theright stereo channel.

The left-hand side of FIG. 10 shows the first user 1001 moving to aposition relative to the second user 1002 such that the users are withina predetermined user-transfer distance 1003 of each other. The apparatus101 may provide for generation of signalling to provide for presentationof one or more of the audio tracks presented monophonically orstereophonically to one of the first and second user to the other of thefirst and second user. In this example, the apparatus 101 provides forstereophonic presentation of the third audio track 203 and the fourthaudio track 204 to the first user 1001, as shown on the right-hand sideof FIG. 10. Thus, the audio tracks presented to the second user 1002have been shared with the first user 1001.

The addition of the third and fourth audio tracks to the audio trackspresented stereophonically to the first user 1001 has caused the audiotrack limit (of 3) to be reached. Accordingly, the apparatus 101 hasceased presentation of the first audio track 201 and the second audiotrack 202 has been shifted to the right stereo channel to make way forthe third and fourth audio tracks 203, 204 that are perceived asentering via the first user's left ear (given the orientation of theheads of the users when within the predetermined user-transfer distance1003). The third and fourth audio tracks are presented on the centre andleft stereo channels respectively.

The second user 1002 continues to be presented with the third and fourthaudio tracks 203, 204.

It will be appreciated that other configurations of the audio tracksshared between users are possible. For example, the slots or channelsused may be preserved between users. Thus, if the first user had theaudio tracks presented on left and centre channels, the second userwould also be presented with the audio tracks on the same left andcentre channels. In one or more examples the audio tracks may berearranged among the channels/slots or preserved in their channels/slotswhen receiving audio tracks from another user. In one or more examples,the listening experience may be reproduced and thus the slots orchannels of a second user corresponding to the slots or channels usedfor stereophonic presentation of audio tracks to the first user may beused when audio tracks are shared from the first user to the seconduser. The apparatus may provide for ceasing presentation of orrearranging of any audio tracks among channels/slots that were alreadystereophonically presented before the first user shared their audiotracks with the second user.

In one or more examples, the apparatus 101 is caused to determinewhether an audio track is shared from the first user 1001 to the seconduser 1002 or vice versa based on a user gesture from one or both users.For example, in this example, the first user 1001 moved towards thesecond user 1002, within the user-transfer distance 1003 or “bumped”their headphones 108 against the headphones 108 of the second user 1002.Accordingly, the user who made the move towards the other is selected asthe user to receive an audio track from the other. In one or moreexamples, the audio track(s) may be received in the other direction.

The example of FIG. 11 shows a further example of sharing of the audiotracks presented monophonically/stereophonically. In this example, thefirst and second users 1001, 1002 approach each other, to within theuser-transfer distance 1003, face to face. The users 1001, 1002 may bumpforeheads or perform a gesture approaching physically bumping foreheads.Based on signalling indicating such a gesture has occurred and the usersare within the predetermined user-transfer distance 1003, the apparatus101 may provide for replacement of all of themonophonically/stereophonically presented audio tracks presented to oneof the users with the monophonically/stereophonically presented audiotracks of the other user. Thus, compared to FIG. 10, in which the audiotracks were added to the other user's monophonically/stereophonicallypresented audio tracks up to the audio track limit, in this example, theaudio tracks presented to one user replace those presented to the otheruser. With reference to FIG. 11, the first user 1001 ismonophonically/stereophonically presented with the first and secondaudio tracks 201, 202. The second user 1002 ismonophonically/stereophonically presented with the third and fourthaudio tracks 203, 204. In this example, the first user 1001 made theface-to-face move towards the second user 1002. Accordingly, the firstuser 1001 is selected as the user to receive the audio tracks from thesecond user 1002. The right-hand side of FIG. 11 illustrates the audiotracks presented to each user after sharing. The apparatus 101 hasprovided for the first and second audio tracks previously presented tothe first user 1001 to cease presentation and be replaced with the audiotracks presented to the second user 1002. Thus, the first user 1001 isnow presented with the third and fourth audio tracks 203, 204.

The stereo channels or slots to which the shared audio tracks areassigned may be based on the stereo channels or slots to which the audiotracks are assigned for the user from which the audio tracks are shared.Thus, the same listening experience may be achieved by the user“receiving” the shared audio tracks as the user sharing them.Alternatively, the apparatus 101 may assign the shared audio tracks tospare audio channels/slots depending on the physical arrangement betweenthe users when performing a particular gesture indicative of the desireto share. Thus, “bumping” ears may provide for sharing of an audio trackto a stereo channel associated with said ear.

In other examples, the one or more of the audio tracks presented to theusers 1001, 1002 may be transferred between users rather than sharedsuch that the audio track ceases to be presented to one user and beginsto be presented to the other user.

In other examples one or more of the audio tracks presentedmonophonically/stereophonically to one user are exchanged with the audiotracks presented monophonically/stereophonically to the other user. FIG.12 illustrates such an example. As in FIGS. 10 and 11, the left-handside of FIG. 12 shows the monophonic/stereophonic presentation of audiotracks before sharing and the right-hand side illustrates post-sharing.Thus, the first user 1001 is presented with the first and second audiotracks 201, 202. The second user 1002 is presented with the third andfourth audio tracks 203, 204. Based on a user gesture in which bothusers are moving towards one another to a position within thepredetermined user-transfer distance 1003, the apparatus 101 providesfor exchange of the audio tracks from one user to the other. Thus,post-sharing, the first user 1001 is presented with the third and fourthaudio tracks 203, 204 and the second user 1002 is presented with thefirst and second audio tracks 201, 202.

To implement such sharing examples, the apparatus 101 may receivesignalling to indicate that other users are nearby or that another useris within the user-transfer distance 1003. The apparatus 101 may providesignalling to indicate which audio tracks are presented to itsassociated user and/or receive signalling to indicate which audio tracksare presented to the other user. On receipt of signalling indicative ofa predetermined sharing gesture being performed by one or both users andthe distance between them being within the user-transfer distance 1003,the apparatus 101 may provide for transfer of the one or more sharedaudio tracks from an apparatus associated with the sharing user to theapparatus associated with the receiving user. In other examples, anidentifier may be transferred between the apparatuses, such as a URL, sothat the shared audio tracks can be retrieved from a different source.In examples where both users perform a sharing gesture to transfertracks between them, the apparatus may require the sharing gesture to beperformed by the users at the same time or at least within a predefinedtemporal threshold of one another. The sharing gesture may be the samegesture for both users or a different gesture. In one or more examples,different gestures may determine the direction of the sharing of audiotracks (e.g. from the first to the second or from the second to thefirst user).

The user-transfer distance may be any appropriate distance such as up to1 metre, 2 metres, 3 metres or more or less.

FIG. 13 illustrates the apparatus 101 providing for visual presentationof graphics, such as augmented reality graphics using the AR apparatus104, to indicate to other users how many or which audio tracks arecurrently presented to the user associated with the apparatus 101. Inthis example, three augmented reality graphics 1301, 1302 and 1303 areoverlaid over a real-world view of the user 1300. Accordingly, theapparatus 101 may provide signalling to the AR apparatus of other usersso that the appropriate augmented reality graphics can be presented.Alternatively, the apparatus 101, based on a request from an ARapparatus of another user, may report the audio tracks currentlystereophonically/monophonically presented to its user for use ingenerating the augmented reality graphics. Further the apparatus 101may, based on one or more reports from other apparatuses provide fordisplay of augmented reality graphics to overlay the user from which thereport was received.from the point of view of the user with which theapparatus 101 is associated.

In the previous examples, the apparatus 101 is configured to provide forstereophonic presentation to one of left, right or centre stereochannels, or, as shown in FIG. 9, for stereophonic presentation of fouraudio tracks each to one of four slots, each slot having a differentspeaker distribution profile. However, in other examples, the apparatusmay determine different audio presentation configurations depending onthe number of audio tracks required to be presented asstereophonic/monophonic audio. For example, for one audio track, theapparatus 101 may provide for presentation monophonically such that theaudio track is presented to both left and right speakers 401, 402. Whenthe presentation of two audio tracks is required, the apparatus 101 maythen switch to providing for stereophonic presentation using two stereochannels—left and right. When the presentation of three audio tracks isrequired the apparatus may then provide for presentation to three stereochannels (left, right and centre) with one audio track assigned to eachstereo channel. When the presentation of four or more audio tracks isrequired the apparatus may then provide for stereophonic presentationusing slots having different speaker distribution profiles. Whenpresentation of more audio tracks is required than there are slots orstereo channels, the apparatus may provide for mixing of audio tracks sothat multiple audio tracks may be provided on the same stereo channel orslot. Thus, the apparatus may be adaptive to the number of audio tracksit needs to present monophonically/stereophonically at any one time interms of the stereo channels/slots used. Alternatively, the apparatus101 may provide a predetermined number of stereo channels or slots thatare individually filled, as required.

FIG. 14 shows a flow diagram illustrating the steps of,

-   -   based on 1401 a first audio track of at least one audio track,        the first audio track audibly presented to the user as spatial        audio such that it is perceived to originate from a particular        location and based on the user being within a predetermined        distance of the particular location;    -   providing for 1402 a change in the audible presentation of the        first audio track to the user from presentation as spatial audio        to presentation as at least one of monophonic and stereophonic        audio.

FIG. 15 illustrates schematically a computer/processor readable medium1500 providing a program according to an example. In this example, thecomputer/processor readable medium is a disc such as a digital versatiledisc (DVD) or a compact disc (CD). In some examples, the computerreadable medium may be any medium that has been programmed in such a wayas to carry out an inventive function. The computer program code may bedistributed between the multiple memories of the same type, or multiplememories of a different type, such as ROM, RAM, flash, hard disk, solidstate, etc.

User inputs may be gestures which comprise one or more of a tap, aswipe, a slide, a press, a hold, a rotate gesture, a static hovergesture proximal to the user interface of the device, a moving hovergesture proximal to the device, bending at least part of the device,squeezing at least part of the device, a multi-finger gesture, tiltingthe device, or flipping a control device. Further the gestures may beany free space user gesture using the user's body, such as their arms,or a stylus or other element suitable for performing free space usergestures.

The apparatus shown in the above examples may be a portable electronicdevice, a laptop computer, a mobile phone, a Smartphone, a tabletcomputer, a personal digital assistant, a digital camera, a smartwatch,smart eyewear, a pen based computer, a non-portable electronic device, adesktop computer, a monitor, a smart TV, a server, a wearable apparatus,a virtual reality apparatus, or a module/circuitry for one or more ofthe same.

Any mentioned apparatus and/or other features of particular mentionedapparatus may be provided by apparatus arranged such that they becomeconfigured to carry out the desired operations only when enabled, e.g.switched on, or the like. In such cases, they may not necessarily havethe appropriate software loaded into the active memory in thenon-enabled (e.g. switched off state) and only load the appropriatesoftware in the enabled (e.g. on state). The apparatus may comprisehardware circuitry and/or firmware. The apparatus may comprise softwareloaded onto memory. Such software/computer programs may be recorded onthe same memory/processor/functional units and/or on one or morememories/processors/functional units.

In some examples, a particular mentioned apparatus may be pre-programmedwith the appropriate software to carry out desired operations, andwherein the appropriate software can be enabled for use by a userdownloading a “key”, for example, to unlock/enable the software and itsassociated functionality. Advantages associated with such examples caninclude a reduced requirement to download data when furtherfunctionality is required for a device, and this can be useful inexamples where a device is perceived to have sufficient capacity tostore such pre-programmed software for functionality that may not beenabled by a user.

Any mentioned apparatus/circuitry/elements/processor may have otherfunctions in addition to the mentioned functions, and that thesefunctions may be performed by the sameapparatus/circuitry/elements/processor. One or more disclosed aspectsmay encompass the electronic distribution of associated computerprograms and computer programs (which may be source/transport encoded)recorded on an appropriate carrier (e.g. memory, signal).

Any “computer” described herein can comprise a collection of one or moreindividual processors/processing elements that may or may not be locatedon the same circuit board, or the same region/position of a circuitboard or even the same device. In some examples one or more of anymentioned processors may be distributed over a plurality of devices. Thesame or different processor/processing elements may perform one or morefunctions described herein.

The term “signalling” may refer to one or more signals transmitted as aseries of transmitted and/or received electrical/optical signals. Theseries of signals may comprise one, two, three, four or even moreindividual signal components or distinct signals to make up saidsignalling. Some or all of these individual signals may betransmitted/received by wireless or wired communication simultaneously,in sequence, and/or such that they temporally overlap one another.

With reference to any discussion of any mentioned computer and/orprocessor and memory (e.g. including ROM, CD-ROM etc), these maycomprise a computer processor, Application

Specific Integrated Circuit (ASIC), field-programmable gate array(FPGA), and/or other hardware components that have been programmed insuch a way to carry out the inventive function.

The applicant hereby discloses in isolation each individual featuredescribed herein and any combination of two or more such features, tothe extent that such features or combinations are capable of beingcarried out based on the present specification as a whole, in the lightof the common general knowledge of a person skilled in the art,irrespective of whether such features or combinations of features solveany problems disclosed herein, and without limitation to the scope ofthe claims. The applicant indicates that the disclosed aspects/examplesmay consist of any such individual feature or combination of features.In view of the foregoing description it will be evident to a personskilled in the art that various modifications may be made within thescope of the disclosure.

While there have been shown and described and pointed out fundamentalnovel features as applied to examples thereof, it will be understoodthat various omissions and substitutions and changes in the form anddetails of the devices and methods described may be made by thoseskilled in the art without departing from the scope of the disclosure.For example, it is expressly intended that all combinations of thoseelements and/or method steps which perform substantially the samefunction in substantially the same way to achieve the same results arewithin the scope of the disclosure. Moreover, it should be recognizedthat structures and/or elements and/or method steps shown and/ordescribed in connection with any disclosed form or examples may beincorporated in any other disclosed or described or suggested form orexample as a general matter of design choice. Furthermore, in the claimsmeans-plus-function clauses are intended to cover the structuresdescribed herein as performing the recited function and not onlystructural equivalents, but also equivalent structures. Thus although anail and a screw may not be structural equivalents in that a nailemploys a cylindrical surface to secure wooden parts together, whereas ascrew employs a helical surface, in the environment of fastening woodenparts, a nail and a screw may be equivalent structures.

1-15. (canceled)
 16. An apparatus comprising at least one processor; andat least one memory including computer program code; the at least onememory and the computer program code configured to, with the at leastone processor, cause the apparatus at least to perform: based on a firstaudio track of at least one audio track, the first audio track audiblypresented to the user as spatial audio such that it is perceived tooriginate from a particular location and based on the user being withina predetermined distance of the particular location; provide for achange in the audible presentation of the first audio track to the userfrom presentation as spatial audio to presentation as at least one ofmonophonic and stereophonic audio.
 17. The apparatus of claim 16,wherein the first audio track is audibly presented as spatial audio byprocessing the audio track using one or more of: ahead-related-transfer-function filtering technique; or avector-base-amplitude panning technique.
 18. The apparatus of claim 16,wherein the provision of the change to monophonic or stereophonic audiois based on the user being within a predetermined distance of theparticular location and providing a predefined user input.
 19. Theapparatus of claim 16, wherein the particular location and the locationof the user is defined relative to one of: a virtual space, the virtualspace comprising a virtual environment in which a representation of theuser is free to move; or a real-world space in which the user islocated.
 20. The apparatus of claim 16, wherein the user is associatedwith at least two speakers, the two speakers located on opposite sidesof the user's head to provide audio to different ears and wherein basedon a side of the head of the user that is closest to the particularlocation, at least within a predetermined time window prior to provisionof the change, defines, at least in part, which one of the at least twospeakers the first audio track is provided to for audible presentationwith a greater volume than the other when the apparatus provides for theaudible presentation of the first audio track as one of monophonic andstereophonic audio.
 21. The apparatus of claim 16, wherein based on theuser being presented with a second audio track as one of monophonic orstereophonic audio, the provision of the change in the audiblepresentation of the first audio track provides for: audible presentationof the first audio track with a first speaker distribution profile; andaudible presentation of the second audio track with a second speakerdistribution profile different to the first speaker distributionprofile; wherein the first and second speaker distribution profilesdefine at least which of a first speaker and a second speaker the audiotrack is presented to, the first and second speakers located on oppositesides of the user's head to provide audio to the user's left and rightear.
 22. The apparatus of claim 21, wherein the second speakerdistribution profile is the same speaker distribution profile used topresent the second audio track prior to audible presentation of thefirst audio track as one of monophonic and stereophonic audio; or thesecond speaker distribution profile is a different speaker distributionprofile used to present the second audio track prior to audiblepresentation of the first audio track as one of monophonic andstereophonic audio.
 23. The apparatus of claim 16, wherein apredetermined audio track limit of two or greater is provided thatlimits the number of audio tracks that can be presented to the user asone of monophonic and stereophonic audio at any one time; and whereinbased on the user being presented with a maximum number of audio tracksas one of monophonic and stereophonic audio that meets the audio tracklimit; and based on the provision of the change in the audiblepresentation of the first audio track, the apparatus is caused toprovide for the ceasing of the audible presentation as one of monophonicand stereophonic audio of at least one of the maximum number of audiotracks.
 24. The apparatus of claim 16, wherein the apparatus is causedto maintain a list of audio tracks for presentation to the user as oneof monophonic and stereophonic audio at any one time, and the audiotracks in the list are user selectable such that on selection of one ofthe audio tracks in the list, the apparatus is caused to provide foraudible presentation of the selected audio track without unselectedaudio tracks in the list as one of monophonic and stereophonic audio.25. The apparatus of claim 16, wherein based on signalling indicative ofa detection of a predetermined head movement of the user, the apparatusis caused to provide for ceasing of audible presentation of at least oneaudio track presented to the user as one of monophonic and stereophonicaudio.
 26. The apparatus of claim 16, wherein first audio trackcomprises one of a plurality of audio tracks, the plurality of audiotracks audibly presented to the user as spatial audio such that each isperceived to originate from a respective particular location, and basedon a user input indicative of a selection of the plurality of audiotracks; provide for a change in the audible presentation of theplurality of audio tracks to the user from presentation as spatial audioto presentation as at least one of monophonic and stereophonic audio.27. The apparatus of claim 16, wherein based the user being within apredetermined user-transfer distance of a second user, provide forgeneration of signalling to provide for one or more of presentation of ashared audio track to the second user as at least one of monophonic andstereophonic audio, the shared audio track comprising an audio trackpresented to the user as at least one of monophonic and stereophonicaudio; or presentation of a shared audio track to the user as at leastone of monophonic and stereophonic audio, the shared audio trackcomprising an audio track presented to the second user as at least oneof monophonic and stereophonic audio.
 28. The apparatus of claim 27,wherein the provision of the signalling is further based on a usergesture from one or both of the user and the second user.
 29. A method,the method comprising based on a first audio track of at least one audiotrack, the first audio track audibly presented to the user as spatialaudio such that it is perceived to originate from a particular locationand based on the user being within a predetermined distance of theparticular location; providing for a change in the audible presentationof the first audio track to the user from presentation as spatial audioto presentation as at least one of monophonic and stereophonic audio.30. The method of claim 29, wherein the first audio track is audiblypresented as spatial audio by processing the audio track using one ormore of: a head-related-transfer-function filtering technique; or avector-base-amplitude panning technique.
 31. The method of claim 29,wherein the provision of the change to monophonic or stereophonic audiois based on the user being within a predetermined distance of theparticular location and providing a predefined user input.
 32. Themethod of claim 29, wherein the particular location and the location ofthe user is defined relative to one of: a virtual space, the virtualspace comprising a virtual environment in which a representation of theuser is free to move; or a real-world space in which the user islocated.
 33. The method of claim 29, wherein the user is associated withat least two speakers, the two speakers located on opposite sides of theuser's head to provide audio to different ears and wherein based on aside of the head of the user that is closest to the particular location,at least within a predetermined time window prior to provision of thechange, defines, at least in part, which one of the at least twospeakers the first audio track is provided to for audible presentationwith a greater volume than the other when providing the audiblepresentation of the first audio track as one of monophonic andstereophonic audio.
 34. The method of claim 29, wherein based on theuser being presented with a second audio track as one of monophonic andstereophonic audio, the provision of the change in the audiblepresentation of the first audio track provides for: audible presentationof the first audio track with a first speaker distribution profile; andaudible presentation of the second audio track with a second speakerdistribution profile different to the first speaker distributionprofile; wherein the first and second speaker distribution profilesdefine at least which of a first speaker and a second speaker the audiotrack is presented to, the first and second speakers located on oppositesides of the user's head to provide audio to the user's left and rightears.
 35. A non-transitory computer readable medium comprising programinstructions stored thereon for performing at least the following: basedon a first audio track of at least one audio track, the first audiotrack audibly presented to the user as spatial audio such that it isperceived to originate from a particular location and based on the userbeing within a predetermined distance of the particular location;providing for a change in the audible presentation of the first audiotrack to the user from presentation as spatial audio to presentation asat least one of monophonic and stereophonic audio.